Normally, filter residues settle at the entry side of the filter applied for filtration, when filtering fluids and these residues thus continuously reduce the filtering capability of the filter. Inasmuch as this is concerned, it is necessary to free the applied filter from filter residues at certain intervals, in particular during a filtering process continuing for a longer period of time. For this purpose, the filters are usually back-flushed with fluid. This back-flushing typically reduces the efficiency of the filtering process, which is particularly the case if a part of the permeate which is produced with the filtration is used for back-flushing the filter.
It is known to admix special pre-treatment agents to the fluid to be filtered, at the entry side of the filter, in order to be able to simplify the release of deposits adhering to the filter when back-flushing, wherein these pre-treatment agents in the fluid effect a precipitation of substances dissolved in the fluid and/or a flocculation of colloidal constituents of the fluid which is to say reduce the binding of the filter residues on the filter.
Different procedural manners are common for metering such pre-treatment agents. A known method lies in admixing a constant flow of pre-treatment agent to the fluid to be filtered. With another known method, the pre-treatment agent is added to the fluid to be filtered such that the concentration of this agent in the fluid to be filtered is kept constant during the complete filtering process. Both methods however have the disadvantage that they have been found to be ineffective if the degree of contamination of the fluid to be filtered changes during the filtration process.
A further approach for metering the pre-treatment agent lies in providing a type of closed control loop and detecting parameters relating to the quality of the fluid to be filtered, by way of sensors. This procedural manner, although representing a certain improvement compared to the provision of a constant quantity or concentration of the pre-treatment agent, however this measure too has been found to be ineffective, since the constraints before and on filtration as a whole cannot be acquired in their entirety by this measure.